Centrifugal impeller

ABSTRACT

A low noise centrifugal fan impeller has a multi cellular form of construction comprising a multiplicity of individual radial flow channels disposed in a circumferential and longitudinal array with respect to the axis of rotation. This increases the “blade passing frequency” as compared to a conventional impeller and since higher frequencies have lower energy for a given flow output the total amplitude of the noise generated can be reduced. Each channel extends from a respective channel inlet to a respective channel outlet which is offset from the respective inlet in the longitudinal direction of the impeller, and involves an increase in the internal dimension of the respective channel in the direction parallel to the axis of rotation, thus assisting deceleration and compression of the air or other medium being conveyed as it flows through the channels.

The present invention relates to rotary impellers used in fluid-dynamic machines for moving and/or compressing gases or liquids or other fluent materials, such as fans, blowers, propellers, pumps and compressors, and more particularly is concerned with the production of low noise levels by such devices. Most machines of this kind fall within one of two main classes, namely axial flow machines where the overall direction of flow of fluid through the impeller is generally parallel to its axis of rotation, and centrifugal machines where the overall direction of flow of fluid through the impeller is generally radial to its axis of rotation. It is to the latter class that the present invention is directed.

The impeller is a significant source of noise in such machines, the noise from the impeller being caused by the repeated exit of fluid from the individual blades (tonal noise) and the turbulent passage of the fluid over the blades (broadband noise). There exists a clear market drive for lower noise machines, beyond the requirements of current legislation. For example low noise levels are used as positive selling points in relation to vacuum cleaners, hairdryers, cooker hoods, air conditioning units and other domestic appliances where noise is dominated by the aerodynamic sources. Similarly there is a demand for quieter cooling fans in compact electronic equipment, particularly computers, where there is already a market for replacement fans with lower noise levels and noise absorption kits. Another market exists for lower noise propulsion units for air and water vehicles. Impellers according to the invention may find application in all of these markets, or more generally in centrifugal machines where there is a desire for lower noise operation.

In a traditional bladed impeller each individual blade compresses a volume of fluid which contributes to the total flow through the machine. The presence of static parts in the vicinity of the impeller, such as the casing outlet port in a centrifugal configuration, causes a disruption to the pressurised working fluid and the resultant pressure wave is evident as noise either in the fluid itself or emanating from the body of the machine. The frequency of noise created in this way is known as the blade passing frequency and is dependent on the number of blades in the impeller and the number of significant static features at the exit from the impeller.

The present invention is predicated upon replacing the traditional bladed impeller with a multi-cellular form of construction with the impeller comprising a multiplicity of individual radial flow channels disposed in a circumferential and longitudinal array with respect to the axis of rotation of the impeller. This dramatically increases the equivalent number of “blades” passing the static features and correspondingly raises the frequency of the tonal noise generated. For a given flow output, higher frequencies have lower energy and hence the total amplitude of the noise generated should be reduced.

A centrifugal impeller having a construction as described above (hereinafter referred to as “an impeller of the kind stated”) is disclosed and illustrated in a schematic manner in US2004/0184914. The present invention seeks, however, to provide an impeller of the kind stated with an enhanced fluid dynamic performance. In this respect the efficient transfer of momentum to the fluid in the rotating impeller requires an increase in the available flow area along the length of the flow channels. In the centrifugal impeller of US2004/0184914 some increase in the available flow area of each channel will inherently be achieved by virtue of the circumferential widths of the channels increasing with increasing radial distance from the axis of rotation. In one aspect of the present invention, however, a further increase in the available flow area can be achieved by configuring flow channels so as to provide an increase in a dimension orthogonal to the circumferential direction.

In one aspect the invention accordingly resides in an impeller of the kind stated wherein at least a plurality of said channels are configured to extend from a respective channel inlet to a respective channel outlet which is offset from the respective inlet in the longitudinal direction of the impeller, and involve an increase in the internal dimension of the respective channel in the direction parallel to the axis of rotation of the impeller. The flow channels may also be so configured that their inlets collectively define an inlet array of dished form surrounding the axis of rotation, the radius of which array decreases with increasing distance in the longitudinal direction of the impeller from a main inlet to the impeller at one end thereof.

The array of flow channels in an impeller according to the invention is preferably in the form of a succession of circumferential rows of such channels arranged along the impeller, with the channels in each such row being offset in the circumferential direction from the channels in the adjoining such row(s). This circumferential offset reduces the number of channels passing the same static feature at any instant and further breaks up the fluid flow which reduces the generation of discrete tones and distributes the noise over a wider range of frequencies.

In a preferred embodiment of the invention the flow channels are tessellated together with cross-sectional shapes based on hexagons (or truncated hexagons in the case of the rows of channels at each end of the impeller). This provides an inherently strong structural form with a high area efficiency for fluid flow, allows a natural offset for the channels in adjoining rows, provides “flat” radial faces for working the fluid, and is more efficient than, say, circular or elliptical channels in terms of structural weight and flow area. This is not an essential feature of the invention, however, and other cross-sectional forms may be adopted if desired. For example a channel form based on quadrilaterals would provide a slightly higher area efficiency at the expense of some structural strength for a given wall thickness, and could therefore be more suited to lower load applications.

The number of flow channels in each circumferential row is preferably the same, and is preferably a prime number to reduce the formation of harmonics of the blade passing frequency in the noise spectrum.

These and other features of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a pictorial view of a preferred embodiment of a centrifugal fan impeller according to the invention;

FIG. 2 is an axial section through the impeller of FIG. 1;

FIG. 3 is a simplified schematic diagram illustrating the increase in “axial thickness” of flow channels in the impeller of FIGS. 1 and 2; and

FIG. 4 illustrates comparative noise spectra measured for a conventional vacuum cleaner when equipped with its original impeller and with an impeller according to the invention.

Referring to FIGS. 1 and 2, the impeller there illustrated is for a centrifugal air fan or blower such as may be incorporated in a vacuum cleaner, hairdryer or the like appliance. It comprises a generally annular structure bounded at one longitudinal end by a disc 1 with a central aperture 2 constituting the main air inlet to the device, and closed at the other end by a disc 3 which extends inwardly in the form of a streamlined body 4 adapted to be fitted onto the spindle of an associated motor (not shown) for rotating the impeller in use. A multiplicity of radial air flow channels 5 extend through the impeller and when it is rotated induce a flow of air to pass in through the aperture 2 and along the channels 5 into an associated conventional volute casing (not shown) whence it is directed as required.

More particularly, in the illustrated embodiment there are a total of 44 channels 5 disposed in an array of four circumferential rows, each comprising 11 channels, arranged along the impeller. The individual channels are of generally hexagonal cross-section, truncated to five sides for the end rows bounded by the discs 2 and 3, and tessellated as shown so that the centres of the channels in each row are offset in the circumferential direction from those in the adjoining row or rows by a half channel width.

Furthermore the centrelines of the channels 5 do not each lie in a single radial plane but as shown particularly in FIG. 2 the channels are swept in the longitudinal direction of the impeller away from the inlet end so that the centres of their individual outlets are offset in that direction from the centres of their respective inlets. By this means a useful increase in the cross-sectional area available for flow through each channel can be achieved along the length of the channel, over and above that which naturally occurs through the circumferential widening of the channel as it extends outwards from the axis of the impeller, thus assisting deceleration and compression of the air as it flows through the channels. This feature of the channel geometry may be more easily appreciated by the simplified representation of FIG. 3 where it is seen that the dimension of each channel 5 in the direction parallel to the axis of rotation of the impeller—i.e. the “axial thickness” exemplified by the dimensions a and b at the inlet and outlet of a typical channel in FIG. 3—increases along the length of each channel.

As also shown in FIGS. 1 and 2, the inlet ends of the channels 5 are swept towards the main inlet 2, with the radius of the ring of channel inlets in each circumferential row decreasing for successive rows away from the main inlet 2, so that the channel inlets collectively define an array of dished form surrounding the body 4. In combination with the profiled centre body 4 this geometry assists in smoothing the transition of the airflow from the axial direction as it enters the inlet 2 to the radial direction as it passes through the channels 5, while maintaining the desired flow area ratios. In the limit each channel could be swept so far that their individual inlets lie in the plane of the aperture 2, resulting in a hybrid form of device which acts as an axial flow impeller at the inlet ends of the channels and transitions to a centrifugal impeller further along the channels.

The channels 5 also have a component of curvature in the circumferential direction to improve compression of the air.

FIG. 4 is a graph which illustrates by the solid line the noise spectrum obtained from a conventional vacuum cleaner when fitted with its original centrifugal impeller (being of conventional design with seven blades and 90 mm in diameter), and by the broken line the noise spectrum obtained from the same appliance when the impeller was replaced with one of the same diameter substantially in accordance with the design shown in FIGS. 1 and 2. In each case measurements were taken with the vacuum cleaner running and placed on the floor, by a sound level meter located 1 m above and 1 m in front of the appliance. By comparing the results it can be seen that the impeller according to the invention eliminated a number of discrete peaks in the conventional spectrum and achieved a reduction in amplitude across practically all of the frequency range to which the human ear is most sensitive.

While a preferred embodiment of a rotary impeller has been described above and illustrated in the accompanying drawings, those skilled in the art will appreciate that numerous variations may be made to the illustrated design, for example in terms of the number, size and fluid dynamic form of the flow channels, to meet specific requirements and applications, without departing from the scope of the invention. 

1. A centrifugal impeller comprising a multiplicity of individual radial flow channels disposed in a circumferential and longitudinal array with respect to the axis of rotation of the impeller, wherein at least a plurality of said channels are configured to extend from a respective channel inlet to a respective channel outlet which is offset from the respective inlet in the longitudinal direction of the impeller, and involve an increase in the internal dimension of the respective channel in the direction parallel to the axis of rotation of the impeller.
 2. An impeller according to claim 1 wherein the flow channels are so configured that their inlets collectively define an inlet array of dished form surrounding the axis of rotation, the radius of which array decreases with increasing distance in the longitudinal direction of the impeller from a main inlet to the impeller at one end thereof.
 3. An impeller according to claim 2 further comprising a profiled body located around the axis of rotation within said dished inlet array, the radius of which body increases with increasing distance in the longitudinal direction of the impeller from said main inlet.
 4. An impeller according to claim 1 wherein the flow channels are so configured that their inlets collectively define an inlet array lying in a generally radial plane with respect to the axis of rotation.
 5. An impeller according to claim 1 wherein at least a plurality of said flow channels are of generally hexagonal cross-section.
 6. An impeller according to claim 1 wherein at least a plurality of said flow channels are of generally quadrilateral cross-section.
 7. An impeller according to claim 1 wherein the array of flow channels is in the form of a succession of circumferential rows of such channels arranged along the impeller, with the channels in each such row being offset in the circumferential direction from the channels in the adjoining such row(s).
 8. An impeller according to claim 7 wherein there are at least three such rows and the flow channels in at least that or those row(s) which are not at the longitudinal ends of the array are of generally hexagonal cross section.
 9. An impeller according to claim 1 wherein the array of flow channels is in the form of a succession of circumferential rows of such channels arranged along the impeller, with the same number of channels in each such row.
 10. An impeller according to claim 9 wherein said number is a prime number.
 11. A fluid dynamic machine incorporating an impeller according to claim
 1. 